Process for manufacturing of brazed multi-channeled structures

ABSTRACT

A process is disclosed for brazing plate/plate and plate/fin multi-channeled structures using an amorphous brazing foil as a brazing filler met between the parts in order to form uniform joints having optimal dimensions, shape and strength. The parts are assembled in an unconstrained stack, and a controlled load is applied to the top of the stack. The stack is then heated to a temperature at which the interlayer melts and reacts with the base metal to form the joints. The stack is cooled resulting in a brazed structure having the desired characteristics, wherein the strength of the structure is equal to the underlying strength of the base metal.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a manufacturing method of brazinguniform plate/plate and plate/fin multi-channeled structures using anamorphous brazing foil as a brazing filler metal.

FIELD OF THE INVENTION

[0002] 1. Description of the Prior Art

[0003] Brazing is a process for joining metal parts, often of dissimilarcomposition, to each other. Typically, a brazing filler metal that has amelting point lower than that of the base metal parts to be joined isinterposed between the parts to form an assembly. The assembly is thenheated to a temperature sufficient to melt the brazing filler metal.Upon cooling, a strong and preferably corrosion resistant joint isformed.

[0004] One class of products produced by brazing processes is the heatexchanger, a three-dimensional structure comprised of alternating metalflat plates and fins or corrugated plates kept in tight, physicalcontact. Brazed joints mechanically connect and seal the contact areasbetween the flat plates and fins, as in the case of plate/fin heatexchangers or between the stamped corrugations in the plate/plate case.The corrugation profiles may have a chevron pattern, pressed-outindentations of various circular forms or some other profiles. In thebrazed state these indentations are joined with flat plates or with eachother forming an elaborate system of channels or interlocking cavities.In service, hot and cool liquids and/or gasses flow separately in thesechannels exchanging heat. In many cases, these structures are made fromheat and corrosion resistant steels and operate at high temperatures ascoolers in utility systems, as heat exchangers in aerospace structures,and as recuperators in chemical, food and other-process industries.

[0005] The majority of multi-channeled brazed structures are producedusing a filler metal placed between the base metal parts prior to theactual brazing process. Filler metal in powder form can be sprayed ontothe surfaces of the base metal parts or applied in the form of apowder/polymer composite paste. In either case, filler metal in powderform is porous and contains considerable impurities in the form ofoxides. The use of powder filler metals in this manner results inuneven, porous and poor quality joints. Alternately, filler metal infoil form can be placed directly between the base metal parts to bejoined. Foil, by comparison, is 100% dense, carries fewer impurities andcan be more accurately metered in the joint area. The use of foil inconstrained assemblies, while being much more effective than powder,necessitates small variations in the foil thickness. This isparticularly important when these assembled alternating base metalplates and foil preforms are constrained from mutual movement duringbrazing.

[0006] To optimize the brazed structure performance, one needs toconduct tedious, preliminary experiments to determine the proper amountof powder or foil relative to the base metal plate thickness andgeometry. Moreover, constrained assemblies require that all parts havevery precise dimensions and a very accurate part placement that isdifficult and expensive to satisfy using the existing technology. Toillustrate, consider corrugated sheet in a plate/plate heat exchangerthat is 250 mm wide by 100 mm long by 0.1 mm thick, with a channelheight of 5 mm. An optimized brazed joint will have a gap of 0.025 to0.050 mm. A 1% variation in the channel height will cause the gap tochange by 0.05 mm. A 1% deflection in the flatness of the sheet willcause the gap to change by 1 mm. The only way to seal these local largegaps is to fill them with brazing filler metal. When the gaps are large,but the amount of available filler metal is small or the filler metalhas poor flow, then filling of the excessive gaps may not be sufficientin a mechanically constrained assembly of plates and preforms. As aresult, there may be large unbrazed areas.

[0007] A properly designed heat exchanger must contain the liquidsand/or gases in their appropriate channels and must safely withstand thepressure exerted upon it by the fluid media. These design criteria applyto each brazed joint in the structure. The joint strength is a parameterdetermined by the joint size and microstructure. It is affected by thetime-temperature brazing conditions. Given the larger number of jointsin a heat exchanger, joint strength and integrity are rather difficultto predict and, even more, difficult to regulate. In the ideal case ofhigh strength joint, a potential failure of the brazed structure underthe critical internal pressure would occur in the structural parts madeof the base metal rather than in the brazed joint.

[0008] Steel heat exchangers are typically brazed with Cu, Ni- orCo-based filler metals. Cu filler metal is available in foil form. Theuse of Cu, however, is limited to heat exchangers that experiencemoderate temperature and contain minimally corrosive media. Ni- andCo-based filler metals produce brazed joints capable of withstandinghigh temperatures and moderately corrosive media. The majority of Ni-and Co-based advanced filler metals that can be used for joining thesestructures contain a substantial amount of metalloid elements such asboron, silicon and/or phosphorus. Consequently, such alloys are verybrittle in conventional crystalline form and available only as powders,powder-binder pastes and tapes and bulky cast preforms. Powders andpowder-based preforms do not easily permit brazing of complex forms.However, these Ni- and Co-based alloys can be transformed into aductile, flexible foil that is produced utilizing rapid solidificationtechnology and which has an amorphous structure in the solid state. Suchamorphous alloys for brazing applications are disclosed in many patents,for example U.S. Pat. Nos. 4,148,973 and 4,745,037. In spite ofsubstantial advantages of rapid solidification technology achieved sofar, the foil thus produced has cross-sectional and longitudinalthickness variations, sometimes exceeding ±40%.

[0009] Thus, there is a continuing need for an improved method ofbrazing complex three-dimensional plate/plate and plate/fin structuresthat can provide strong joints with controlled cross-section dimensionswithout being overly dependent on: (a) brazing foil thickness and itsvariations; and (b) the shape and accuracy of dimensions of fins andprofiles.

[0010] There exists experimental data showing the beneficial effect ofload applied normal to the joints of specimens subjected to brazingoperations. In each of these specimens, amorphous metal foil was used asthe brazing filler metal. The brazed joint thickness varied with theapplied load. This data indicates the importance of load in theimprovement of liquid filler metal wetting of rough gap surfaces andformation of non-porous brazes. Moreover, the self-adjusting interplaybetween the surface tension of a liquid filler applied load alsooptimizes the thickness, the microstructure and, most importantly, thestrength of the brazement. This fundamental load effect provides thescientific basis for the proposed method of the present invention toimprove brazed multi-channeled structures.

SUMMARY OF INVENTION

[0011] This invention is embodied in a brazing method comprising thesteps of interposing an interlayer in an amorphous foil form betweenplates and fins to be joined, assembling parts in an unconstrainedstack, applying a controlled load on the top of the stack, heating theassembly under suitable conditions to a temperature at which theinterlayer melts and reacts with the base metal parts, and cooling theassembly to produce a structure with uniform joints having optimaldimensions, shape and strength.

[0012] The invention also comprises a brazed structure produced by themethod described hereinabove.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Details of the invention, and of certain preferred embodimentsthereof, will be better understood upon reference to the drawings,wherein:

[0014]FIG. 1(a) is a top view of a circular joint formed in a brazedheat exchanger. FIG. 1(b) is a top view of an elongated joint formed ina brazed heat exchanger.

[0015]FIG. 1(c) is a cross-sectional view of the joints shown in FIG.1(a) and in FIG. 1(b) formed in a brazed heat exchanger.

[0016]FIG. 2(a) is a schematic view of a plate/fin assembly before andafter brazing.

[0017]FIG. 2(b) is a schematic view of a plate/plate assembly before andafter brazing.

[0018]FIG. 3(a) is a photograph of a sample used to demonstrate anembodiment of the invention before mechanical testing at 650° C.

[0019]FIG. 3(b) is a photograph of a sample used to demonstrate anembodiment of the invention after mechanical testing at 650° C.

[0020]FIG. 4(a) is a photograph of the microstructure of a stainlesssteel joint brazed using 25 μm thick amorphous foils.

[0021]FIG. 4(b) is a photograph of the microstructure of a stainlesssteel joint brazed using 50 μm thick amorphous foils.

[0022]FIG. 5(a) is a photograph showing a side view of a sample afterfailure under tensile mode mechanical testing at 650° C. showing thefailure location in the brazed joint.

[0023]FIG. 5(b) is a photograph showing a side view of a sample afterfailure under tensile mode mechanical testing at 650° C. showing thefailure location in the base metal.

[0024]FIG. 6 is a photograph showing a view of two halves of a samplemanufactured with a thin (25 μm) preplaced amorphous brazing fillermetal in a foil form showing a large unbrazed area due to presence of asmall dent in the fin.

DETAILED DESCRIPTION OF THE INVENTION

[0025] To guarantee a leak resistant product, brazed multi-channeledstructures must have complete brazements at each contact surface. Inaddition, to assure extended service life, these brazements must beuniform and strong.

[0026] The mechanical performance of the device is characterized bythree primary parameters: 1) the maximum burst pressure that can beresisted by the brazed structure, 2) the long-term dimensional stabilityof the structure against sustained pressure, and 3) the long-termmechanical stability of the structure against variable pressure andtemperature. The device may fail at either the brazed joints or withinthe base metal structural members. If the device fails at the brazedjoints, the strength is determined by the total contact surface ofjoints and the strength of the brazements. If it fails in the base metalstructural members, the strength is determined by the cross-section areaand the intrinsic strength of the structural members. These regions inthe device are illustrated in FIG. 1.

[0027] In accordance with the invention, a method is provided tomanufacture a uniform, non-leaking, strong, multi-channeled plate/finand plate/plate brazed structure in which an amorphous foil can be usedas a filler metal. The method consists of the following steps:

[0028] (a) a filler metal in foil form with an amorphous structure andwith a melting temperature that is less than the melting temperature ofthe base metal plate and fin parts is chosen;

[0029] (b) plates 1, fins 2 and brazing filler metal preforms 3 areassembled within a supporting device 5 according to the sequences shownin FIG. 2, such that they can move freely in the vertical direction butare prevented from moving laterally during the brazing operation;

[0030] (c) a compressive load is applied to the assembly by placing afixed amount of weight on the top of the stack as shown in FIG. 2. Thecompressive loads should not cause the stack to collapse at the brazingtemperature;

[0031] (d) the assembly is placed in a furnace and is heated to at leastthe melting temperature of the filler metal; and

[0032] (e) the assembly is cooled.

[0033] Under these conditions, the brazing filler metal melts and fillsthe initial gaps between parts being brazed, as shown in FIG. 2 “AfterBrazing.” Simultaneously, and most specifically, the applied loadadjusts the gaps of each individual brazement being formed to about 15μm to 30 μm, depending on the load. This occurs by moving the base metalparts either closer together or further apart until movement stops dueto equilibrium between the applied load and the surface tension forcesof the molten metal in the gaps. Excessive molten metal is partiallyexpelled out of the brazed gaps forming fillets with largecross-sections and resulting in high strength joints. The overalldimensions of the brazed structure are controlled regardless ofvariations in thickness of foils used and the uniformity of theplate/fin. Moreover, all formed channels of the structure attain equalcross-sections.

[0034] Foils useful in the process described above typically are onaverage about 37 to about 60 μm thick, which is also the desired spacingbetween parts to be joined. Such spacing maximizes the strength of thebrazed joints. Thinner foils may result in insufficient amounts ofliquid filler metal to fill all potential excessive gaps. Thicker foilsare not economical and may not be needed because the failure of thestructure described in the present invention would occur not in thebrazed joints but rather in the base metal parts. Accordingly, the idealperformance of the brazed structure is achieved wherein the failure isdetermined by the strength of the base metal.

EXAMPLE

[0035] In order to illustrate the forgoing, samples were supplied thatwere manufactured in accordance with the following general concepts ofthe present invention. Flat plates were stamped and sinusoidal shapefins were formed from UNS4360 stainless steel sheets having 100 μm and50 μm thickness, respectively. An abrasive water jet cutting method wasused to cut flat filler metal preforms in a foil form from Ni-basedamorphous alloy within American Welding Specification ANSI/A5.8 forBNi-2.

[0036] Amorphous foils of 25 μm, 37 μm, and 50 μm average thickness wereused but their across-the-web profiles, measured by a profilometer witha thin tipped probe, had local troughs as deep as 15-20 μm. Threesamples were assembled as stacks of 16 identical part sets. Each setconsisted of the plate/preform/fin/preform/plate parts. Upon brazing,each of the 16 sets became a plurality of sealed channels simulating thechannels in actual heat exchangers. Each sample was comprised ofidentical base metal plate and fin members for all samples but containedfoil preforms having one of the above mentioned thicknesses. Each samplewas assembled between vertical guides attached to a thick plate of aspecial holder permitting all stack parts to move freely in the verticaldirection during a complete brazing cycle. A load was placed on the topof each sample in the form of a metal or graphite block as shown in FIG.2. The loaded samples in structure 5 were placed in a vacuum furnace inthe vertical position and brazed at a temperature of approximately 1090°C. for 15 min. After brazing, the samples were prepared for mechanicaltesting. They were cut and then machined by the electrical dischargemethod into specimens with I-beam shapes having about a 25 mm×25 mmcross-section in the specimen “neck,” as depicted in FIG. 3(a). Thecutout pieces were used to prepare metallographic samples. The jointdimensions and microstructure, as a function of the preform thickness,were measured using an optical microscope under a moderate 100×magnification.

[0037] Metallographic observations showed that the joint thickness inthe middle portion of all brazes is the same regardless of the thicknessof the virgin amorphous foil, even when comparing samples manufacturedusing 25 μm and 50 μm foils as FIGS. 4a and 4 b demonstrate. This effectwas observed because brazing gaps were not constrained. Indeed, theexcess liquid MBF-20 alloy was partially expelled from the capillarygaps upon melting until the surface tension forces at all gap surfacesbecame equal to the total load applied to the specimen, the total loadbeing the parts weight and the weight of the external block. Thisexcessive molten MBF-20 metal, particularly in the 50 μm foil, flowedout of the initial gaps forming large fillets and partially climbed upon the vertical walls of the fins. The thicker filler metal resulted inlarger joint fillets which had advantageous shapes without a narrowcavity-like crystallization shrinkage pattern seen in FIG. 4a and,therefore, larger joint cross-sections as depicted in FIG. 4b.

[0038] The height of formed individual passages in all brazed specimenswas measured using a standard optical comparator with the followingresults: Filler metal Average Total height of thickness, passage height,16 passages, mm (mil*) mm (mil*) mm (mil*) 25 μm 3.282 (129.21) 52.514(2067.5) 37 μm 3.287 (129.4)  52.590 (2070.5) 50 μm 3.284 (129.3) 52.557 (2069.2)

[0039] Because sixteen brazing foils were preplaced in each of thesamples, the initial difference between assembled packs with 25 μm and50 μm thick foils was 0.4 mm. This difference decreases to near zero inthe brazed structures. The total difference in heights of these twosamples in the brazed state is only 0.042 mm.

[0040] The I-beam shaped brazed samples were tensile tested at 650° C.using a standard tensile testing machine. The samples evidenced thefollowing maximum load at failure, and this load varied linearly withthe foil thickness: Filler metal thickness, Maximum load at sample mm(mil*) failure at 650° C., kg (lbf*) 25 μm 342 (754)  37 μm 429 (946) 50 μm 537 (1183)

[0041] Optical observations of the failed samples under a moderate 20×magnifi-cation, as depicted in FIGS. 5a and 5 b, showed that in samplesbrazed using 25 μm and 37 μm average thickness foils, the failureoccurred in the brazements, as depicted in FIG. 5a. Also, in somesamples brazed using 25 μm foil, large unbrazed spots were observed dueto an insufficient amount of brazed filler metal needed to filloccasional dents or other defects in the fin form, as FIG. 6demonstrates. In the case of the 50 μm foil sample, the failure occurredin the middle of the fins, as depicted in FIG. 5b. Therefore, in thiscase the strength of the brazed structure was determined ideally by thestrength of the base metal.

[0042] Having thus described the invention in rather full detail it willbe understood that such detail need not be strictly adhered to but thatvarious changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the invention as defined bysubjoined claims.

We claim:
 1. A method for brazing parts comprising the steps of: a)interposing a metallic interlayer in amorphous foil form between theparts to be joined; b) assembling the parts in an unconstrained stack;c) applying a controlled load on the top of the stack; d) heating thestack to a temperature at which the interlayer melts and reacts with thebase metal; and e) cooling the stack to produce a structure with uniformjoints having optimal dimensions, shape and strength;
 2. A methodaccording to claim 1, wherein the parts are selected from the groupconsisting of plates and fins.
 3. A method according to claim 2, whereinthe plates and fins are each made from metals.
 4. A method according toclaim 1, wherein the interlayer is comprised of flat preforms made ofamorphous metal foil.
 5. A method according to claim 4, wherein theamorphous metal foil preferably has an average thickness between about25 μm to about 60 μm, and more preferably between about 37 μm to about50 μm.
 6. A method according to claim 1, wherein the value of the loadapplied does not exceed the value which will cause the stack to collapseat the brazing temperature.
 7. A method according to claim 1, whereinthe uniform brazed joints are of an optimal thickness and havefull-bodied fillets formed to completely seal all brazing gaps withoutresidual pores.
 8. An article manufactured in accordance with the methodof claim
 2. 9. An article according to claim 8, wherein the article ismade from a material selected from the group consisting of series 300stainless steels and series 400 stainless steels.
 10. An articleaccording to claim 8, wherein the article is made from a materialselected from the group consisting of Inconel and Hastelloy superalloys.11. An article according to claim 8, wherein the article is made from amaterial consisting of heat resistant, high chromium, low carbon steels.12. An article according to claim 8, wherein the article is made from amaterial consisting of powder metallurgy, dispersion-hardened alloyscomprising iron, chromium, aluminum and yttria.
 13. An article accordingto claim 8, wherein the article is made from a material consisting oflow alloyed, low carbon steels.
 14. An article according to claim 8,wherein the strength of the article is equal to the strength of the basemetal of the article.
 15. A brazed structure comprising: a) a pluralityof parts; and b) an amorphous foil metallic interlayer between saidparts forming uniform brazed joints having optimal dimensions, shape andstrength; wherein the strength of the structure is equal to the strengthof the underlying material of the parts.
 16. A structure according toclaim 15, wherein the parts are selected from the group consisting ofplates and fins.
 17. A structure according to claim 16, wherein theplates and fins are each metals.
 18. A structure according to claim 15,wherein the amorphous metal foil preferably has an average thicknessbetween about 25 μm to about 60 μm, and more preferably between about 37μm to about 50 μm.
 19. A structure according to claim 15, wherein theuniform brazed joints of an optimal thickness and have full-bodiedfillets are formed to completely seal all brazing gaps without residualpores, resulting in a structure that resists leakage.
 20. A structureaccording to claim 15, wherein the parts are made from a materialselected from the group consisting of series 300 stainless steels andseries 400 stainless steels.
 21. A structure according to claim 15,wherein the parts are made from a material selected from the groupconsisting of Inconel and Hastelloy superalloys.
 22. A structureaccording to claim 15, wherein the parts are made from a materialconsisting of heat resistant, high chromium, low carbon steels.
 23. Astructure according to claim 15, wherein the parts are made from amaterial consisting of powder metallurgy, dispersion-hardened alloyscomprising iron, chromium, aluminum and yttria.
 24. A structureaccording to claim 15, wherein the parts are made from a materialconsisting of low alloyed, low carbon steels.